Conventional ram type blowout preventers (BOPs) are designed to seal full working pressure from one direction only. In such BOPs, pressure below the rams assists the sealing mechanism by pushing the rams together. Wellbore pressure is allowed behind the rams and the wellbore pressure assist-force is equal to the unbalanced area of the ram times the wellbore pressure. The unbalanced area of the ram is that area of the ram's crossection that has wellbore pressure on one side and not the other. Generally, the midline of the ram is the approximate demarcation line of the balanced and unbalanced zones of the ram when sealing from below. Also, the force resulting from the wellbore pressure acting over the cross sectional area of the piston rod must be subtracted from the wellbore assist force.
When attempting to seal from above with a ram-type BOP, wellbore pressure is prevented from getting behind the rams by the rear (i.e., top) seal. The resulting unbalanced force ((wellbore pressure).times.(ram unbalanced cross sectional area)) acts in the direction tending to force the rams apart and reduce the ram's ability to seal. Depending on the actuator size and ram design, some pressure can be held from above, but typically it is only a fraction of the rated working pressure of the BOP. Others have suggested a BOP which will hold a pressure from above the rams, but such a BOP apparently requires an oversized operator to hold the pressure. An exception to the general rule regarding the holding of pressure from above without a large operator applies to low pressure rated (less than 3000 psi) small bore (less than 3.06" diameter) BOPs where it is practical to apply ram closing force that may be adequate to overcome the wellbore force trying to open the rams and thereby seal full working pressure from above or below.
The wellbore assist principle is the basis for all ram-type drilling and coiled tubing BOPs as well as most wireline BOPs (those not falling within the exception in the preceding paragraph). Without wellbore assist, the BOP's actuators would have to be truly massive resulting in a number of design problems and impracticalities.
In those instances where sealing from above is a requirement (usually to hold pressure for a test of equipment higher in the stack), a ram or the entire BOP is often inverted. This inverted BOP is then no longer suitable as a barrier for downhole pressure. A true bi-directional sealing ram type BOP would eliminate the necessity for an additional inverted BOP, replacing it with a single BOP to contain either well pressure from below, or test pressure from above the BOP.
Bi-directional sealing rams have been proposed that would include a 360 degree rear seal (as opposed to the conventional 180 degree seal) and a set of check valves that would let pressure behind the rams regardless of the point of origin of the assist pressure (from above or below). The resulting wellbore assist force would adequately force the rams together and effect a seal at any standard working pressure. However, since the pressure is locked in by the check valves, an arrangement of this sort allows no provision for evacuating the volume behind the ram of pressure short of a manual dump valve, an arrangement that is unacceptable and possibly dangerous. Pressure would have to be manually dumped prior to opening the BOP and would have to be directed to the low pressure side of the ram (maybe above, maybe below depending on circumstances) requiring a valve manifold system to be in place. There are many undesirable features to this concept, not the least of which is the great potential for human error and injury. in addition, pressure behind the ram is trapped at the highest pressure encountered, thus applying undue high stress on the sealing elements when the pressure is reduced.
Thus, there remains a need for a bi-directional ram-type BOP in which pressure-assist is applied to the backs of the rams, and the pressure cannot be trapped behind the rams, regardless of whether it is the top or the bottom of the BOP which is at the higher pressure.